In IC technology there is a need to stack the chips (dies) together to form multi-tiered or three-dimensional (3-D) IC devices. One result of such 3-D IC stacking is a decrease of signal propagation time during signal processing, due to the reduced distance signals must travel when they remain within a single package.
One method for tier bonding is to bring two (or more) dies together and then encapsulate the dies into a single structure. Electrical conductors and/or contacts on the surface of the respective dies serve to carry electrical signals between components on the different dies.
One problem when dies are positioned in close proximity to each other is that thermal density increases. Moreover, because of the reduced size of stacked ICs, (substrate thickness going from 700-100 microns to below 20 microns), the lateral thermal conductivity is reduced. Thus, hot spots could exist with little ability to move the heat away from the heat source.
One method for increasing lateral thermal conductivity is to increase the substrate thickness of at least one of the tiers. Another method is to increase the metal layers in the chip to be able to dissipate heat This, in turn, negatively impacts the desired aspect ratio of the package, as well as degrades the signal transmission speeds.
An additional problem exists when more than one tier is bonded. In such situations, the stacked IC device contains multiple layers of oxide between tiers. Oxide, being a poor thermal conductor, adds to the heat dissipation problem.
There are several approaches that could be taken to help with the thermal conductivity issues. One such approach could use through-silicon-vias (TSVs) to move the heat from an inner portion to a surface layer, and then remove the heat using traditional methods, such as a high thermal conductivity material positioned on a surface of the IC package. A problem with such a solution is that a circuit layout may prevent positioning a TSV at the necessary location because of devices constructed in the various tiers in proximity to the hot spot generating the heat.
Another method is to circulate cooling material through the stacked IC device to cool the various hot spots. This is costly to manufacture because the moving liquid requires a pumping mechanism and tight tolerances for liquid channels. Also, it may not be possible to channel the cooling material to the necessary location. The channeling problem could be overcome, to some extent, by forcing cooling liquid through the substrate itself, but this method is not without a further set of problems and costs.